The present invention relates to a shadow mask used in a color cathode ray tube and more particularly to an aperture grill having vertical slits, and to a method of producing an aperture grill of the above type having a thickness of 100 .mu.m or less.
As a material for aperture grills, cold-rolled low-carbon rimmed steel plate, cold-rolled low-carbon aluminum killed steel plate or low-carbon Fe--Ni invar (36% Ni--Fe alloy) have heretofore been used because these materials are suitable from the viewpoint of being able to be fabricated on an aperture grill, etched and formed into a shape adapted for being built into a cathode ray tube. There are many kinds of shadow masks which are different in the shape of the apertures or openings, in the way of building into the cathode ray tube and in the way of processing during the fabrication. One of the above mentioned materials has been used depending upon the kind of shadow mask. In general, a low-carbon Fe--Ni invar of low coefficient of thermal expansion or a cold-rolled low-carbon aluminum killed steel plate has been used for shadow masks of the slot type and circular hole type. Particularly, the low-carbon Fe--Ni invar of low coefficient of thermal expansion has recently been used with a view to avoiding color deviation that occurs in the cathode ray tube due to thermal expansion when the tube is put into operation.
In the case of shadow masks of the slot type or circular hole type, there is a problem of curling that occurs during the etching process due to a difference in relieving of residual stresses in the regions of the front and rear side openings of the slots or holes because of the difference in the dimension or diameter of the front and rear side openings, whereas in the case of aperture grills such a problem of curling does not occur. Aperture grills are fit into a cathode ray tube in a different way from the other types of shadow masks, and moreover can be formed without plastic deformation in a press. For the above reason, the cold-rolled low-carbon rimmed steel plate has been principally used heretofore for aperture grills.
Aperture grills have heretofore been produced, using a low-carbon steel plate such as a cold-rolled low-carbon rimmed steel plate of a thickness of more than 100 .mu.m. A method of producing an aperture grill was to carry out concurrent etching of a low-carbon steel plate on the opposite surfaces thereof to produce through slits. This method is called a one-step etching method.
Another method of producing an aperture grill is as follows. That is, a low-carbon steel plate is applied with photosensitive resin layers or resist layers on the opposite front and rear surfaces thereof, and then pattern masks are applied to the opposite resin layers. A front pattern mask has one broad slit pattern and a back pattern mask has a narrow slit pattern. Subsequently, the front and rear pattern masks are printed to the front and rear resist layers, respectively, by exposure to light, and then developments on the resist layers of the printed front and rear patterns are made.
A half-etching is carried out on the rear surface of the steel plate through the developed rear resist layer to form a narrow rear recess in the rear surface of the plate; then an etchant-proof resin is filled into the rear recess and over the rear resin layer on the steel plate; and a broad front recess is etched in the front surface of the steel plate through the developed front resist layer to cause the front recess to reach the half-etched rear recess, whereby a through hole is produced in the steel plate. This method is a two-step etching method.
Another method for producing a shadow mask is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 5-12,996 published Jan. 22, 1993, which corresponds to U.S. Pat. No. 5,348,825. In this method, a steel plate is applied with resist layers on the front and rear surfaces thereof, and then a pattern slit mask is applied to only the front surface and printed by exposure to light, while the rear resist layer is maintained as it is and backed up by a backup resin sheet. Etching is carried out on only the front surface of the steel plate through the printed and developed front resist layer to produce a front recess in the steel plate. The front recess reaches the rear resist layer, whereby a through hole is produced in the steel plate when the rear resist layer is removed together with the backup resin sheet. This is a one-side etching method.
The two-step etching method mentioned above, however, takes time and is not efficiently carried out. The one-side etching method referred to above is not sufficient in producing tapered side walls of each slit so as to have a required exact configuration or shape. For those reasons the one-step etching method has generally been used for producing aperture grills.
In the case of the cold-rolled low-carbon rimmed steel plate, residual stress is usually about 10.0 Kg/mm.sup.2 or more when it is subjected to an etching process, but the rimmed steel plate can be used as it is without any particular problems where the plate thickness is more than 100 .mu.m. More specifically, problems have not occurred by taking measures such as to make the direction of the tapes of the aperture grill to be produced by etching of a hoop or band steel, perpendicular to the direction of rolling of the band steel, or to make the direction of the tapes coincide with the direction of rolling of the band steel while tensioning the band steel appropriately. The above measures can prevent the generation of "streaks" in the tapes of the aperture grill. The streaks are produced due to relieving of residual stresses as a result of breakthrough of the slits between the front and rear surfaces of the steel plate during the etching step. For the cold-rolled low-carbon aluminum killed steel plate, the residual stress is generally more than 10.0 Kg/mm.sup.2 as in the case of the cold-rolled low-carbon rimmed steel plate.
In recent years, CRT display devices such as color televisions are becoming enlarged in size, so that shadow masks used in such devices are required to be of large size as well. Particularly, in aperture grills, the manner of fixing the same is different from the manner of fixing of other types of shadow masks having slots or circular openings. That is, the aperture grill is fixed under tension to a rigid frame. For this reason the frame must necessarily be enlarged in relation to the enlarged aperture grill and is required to resist the tension necessary for the fixing of the aperture grill of the conventional thickness. As a consequence, the weight of the frame is increased remarkably. In order to cope with this increase in the weight of the frame, the weight of the aperture grill must be reduced so that the thickness of the aperture grill will have to be reduced to compensate for the enlargement of the size.
Thus the thickness of the material for producing aperture grills should not exceed 100 .mu.m. The above stated measures have been able to solve the problems mentioned above for the material of a thickness of more than 100 .mu.m. Contrary to the case where the material is more than 100 .mu.m thick, the material or hoop steel of a thickness of 100 .mu.m or less causes the following problem. That is, if the material or hoop steel were subjected to etching in such a state that the direction of the tapes of an aperture grill into which the material is manufactured are perpendicular to the rolling direction of the hoop steel, conveying rolls or shafts of the conveying system for the hoop steel would deform the hoop steel because it is thin. Therefore, the above measure which has been employed for thicker hoop steels is not usable. The heretofore used second measure of making the direction of the tapes coincide with the rolling direction while tensioning the steel was found to be also not usable when the one-step etching method is employed because streaks of the tapes occur due to relieving of residual stress when the slits penetrate the steel plate during the etching process. Under the circumstances, etching methods usable for hoop steel plates of a thickness 100 .mu.m or less, to be manufactured into large-size aperture grills, have been limited to the time-consuming two-step etching method that requires reinforcing means, as well as to the one-side etching method that involves the difficulty in obtaining required exact shape of the tapered side walls of the slits.
As above stated, the one-step etching method has advantageously been used heretofore, but this method is disadvantageous for use in etching of a thin steel plate having a thickness of 100 .mu.m or less because of the generation of streaks due to relieving of the residual stress at the time of penetration of the slits through the steel plate.
The streaks are produced due to non-uniformity of stress distribution in the aperture grill tapes. The non-uniform stress distribution causes twisting of each tape, which appears remarkably when the thickness of the steel plate is below 100 .mu.m. It is known that streaks cause variations in the quantity of light that passes through the aperture grill and consequently cause degradation of the quality of images formed on the cathode ray tube in which the aperture grill is fitted.